1. Field of the Invention
The present invention relates to starting circuits for self-biased linear integrated circuits and, in particular, to starting circuits which draw substantially no power after a voltage supply has reached a desired operating level.
2. Description of the Prior Art
Many analog circuits are designed to be self-biased in order to achieve independence from variations in the supply voltage used to energize the circuit. However, such self-biased circuits often have a stable state in which zero current flows in the circuit even when the power supply voltage is nonzero. This occurs because at the instant the power is applied, the currents in the bipolar transistors in the circuits are in the picoampere range. As is known, the current gain of these transistors at very low current levels is often less than unity. As a result, even with the power supply voltage at a nonzero value, the self-biased circuit is usually unable to drive itself out of the zero-current state. Accordingly, unless precautions are taken, the circuit may remain in this undesired zero-current condition, resulting in the circuitry which it biases also remaining nonfunctional.
To avoid the zero-current conditions, separate starting circuits are commonly used to inject an initial current into the self-biased circuits. Since self-biased circuits with zero-current and nonzero-current operating points have sufficient regenerative or positive feedback to bring the biasing currents to the desired nonzero-current operating point, the initial injection of a relatively small current is sufficient to activate the biasing circuit.
Once the supply voltage has had sufficient time to build up to an operating level and the regenerative action of the self-biased circuit has completed the transistion of the biasing currents to their desired operating levels, the starting circuit is no longer needed. Also, the starting circuit must not interfere with the normal operation of the steady state biasing circuit currents once they have reached their desired operating levels. Accordingly, starting circuits are typically designed to automatically disconnect themselves from the biasing circuit as the desired operating point is reached.
While the starting circuits found in the prior art generally provide sufficient starting current for the biasing circuit and adequately switch themselves out of the circuit once the desired operating point has been reached, they have several limitations. A typical starting circuit of conventional design consists of a series resistor-diode combination connected between the positive and negative voltage supply terminals, with the resistor-diode combination connected to the biasing circuit by another diode. Starting current flows through the connecting diode into the biasing circuit until it is biased off by the current buildup in the bias circuit. Such a resistor-diode circuit, however, even when biased off, results in the resistor dissipating power. This power loss is undesirable, especially in the case of a micropower circuit, in which it may result in a significant degradation of the circuit's performance. Furthermore, the resistors used with this technique are usually fairly large and are implemented in integrated circuit form as an epitaxial resistor or a series of pinch resistors. Because of the large resistance value and high rated breakdown voltage necessary, a large area on the chip must be dedicated to the starting circuitry, even though it is used only when power is first applied.